Mycoplasmas commonly produce diseases of considerable economic impact, yet little information is available concerning mechanisms of pathogenesis and effective methods of control are unavailable. Many mycoplasmas possess surface proteins with regions of identical tandem repeats, sometimes with 40 or more repeat units per protein. Such molecules are usually lipoproteins with the lipid moiety serving to anchor the protein to the membrane and the carboxyl tandem repeat region free to interact with host cells and molecules. These proteins are all subject to high-frequency size variation due to slipped-strand mispairing that result in gain or loss of repeat units. The functions of these repetitive proteins and the phenotypic consequences of size variation are unknown. In the murine pathogen Mycoplasma pulmonis, size variation in the Vsa proteins modulates the growth properties of the mycoplasma (colony size variation), hemadsorption, and susceptibility to complement lysis. Our long-range goals are to understand the pathogenic mechanisms of mycoplasmas. The goal of the current proposal is to study the functions and pathogenic significance of Vsa size variation. Many mycoplasmas including M. pulmonis possess an extracellular matrix. We propose that this matrix is composed of the Vsa proteins and serves to protect the mycoplasma from complement. Aim 1 is to examine the matrix by electron microscopy,to determine if it is composed of the Vsa proteins, and to determine whether it is a barrier to complement. The Vsa proteins may affect interactions between the mycoplasma and host cells, and whether Vsa modulates the susceptibility of the mycoplasma to phagocytosis will be examined in Aim 2. In Aim 3, the pathogenic significance of Vsa size variation will be studied by comparing the ability of cells that produce a large (many tandem repeats) or a small (few repeats) Vsa protein in respect to their ability to colonize and cause disease in immuno-competent and complement- deficient mice. The results of these studies will elucidate the role of highly repetitive proteins in mycoplasmal diseases. They will also provide insight into the functions of repetitive proteins in other microbes that are important to public health including the agents responsible for malaria, tuberculosis, and a variety of bacterial pneumonias. By understanding the role of repetitive proteins in the disease process, it is anticipated that improved measures to control the infections of these significant human pathogens will be developed.